Few things spook casual flyers like a bi-fold door. They kick it, or they poke it, Maybe they bang on it, or perhaps yank it with too much gusto. All to do something simple — to get into the bathroom.

It’s hard to blame them. The door lacks a traditional knob, requiring passengers to either push, or pull, at the center of it before they can enter.

“Where do you ever see that in life?” said Scott Savian, an executive vice president at  Zodiac Aerospace, a company that manufactures nearly everything passengers use on the inside of an aircraft, including seats, overhead bins, and bathrooms. “You woke up on the middle of the flight and you’re going through a dark cabin and you’ve got to figure out how to open this door?”

The results from passenger abuse could be disastrous. But even though the average airplane bathroom door looks flimsy, each is generally hardy — built to withstand (almost) everything jet-lagged, drunk, or merely sleep-deprived passengers can do to it. At its test lab, Zodiac has machines that simulate everything passengers might do over an aircraft’s lifetime. One test door might go through more than a million cycles before it’s deemed worthy to fly.

Open. Closed. Open. Closed.

“We try to imagine how they will be abused by passengers and crew potentially,” Savian said. “And then test for worst case scenarios.”

For most products passengers will touch, Zodiac and its competitors go through a similar exercise. It’s important because, if a door or a bin breaks, an airline might have to delay the flight so mechanics can fix it, costing the carrier money when passengers miss their connections. If a seat breaks, the flight can usually go, but an airline can’t sell it.

“Typically a business class seat is going to bring in revenues of somewhere around a million dollars per year,” said Claire Nurcombe, head of marketing for cabin interiors at Stelia Aerospace, a business class seat-maker. “If you take that out of service for a couple of flights until you get the pieces that you need to repair it, it starts to add up quite quickly.”

Avoiding repairs is tricker than it sounds. Airlines want strong finishes, but they don’t want to compromise on cost, weight or efficiency.

Manufacturers try to satisfy their customers, because aircraft interiors is ruthlessly competitive, with many companies fighting for the same business. Executives from one interior provider sometimes criticize another, saying their competitors’ product, while impressive, is not reliable or efficient. Zodiac often takes the brunt of it, as it has had several reliability issues in recent years, including on products for JetBlue Airways, American Airlines and Cathay Pacific, but Savian said it is improving. “This is the number one priority,” he said.

Requests from customers is the reason airplanes have so many bi-fold doors for bathrooms, even though designers prefer to make their work so intuitive no one can improperly use it. Airlines could install easier-to-use doors with handles that open outward — some planes have them now — but when they open they can impede food and drink trolleys, and possibly whack passengers in the face, so they’re not always feasible in aircraft with narrow aisles.

It’s similar with seats. Airlines want the lightest viable seats they can buy, and almost no carrier wants anything heavier than 9 kilograms, or about 20 pounds.

“You could make an indestructible seat,” Savian said. “That’s not the issue. It’s to make a seat at the weight required. “The weight tends to whittle away at the robustness.”

Still, seat makers say they get close to making products no one can ruin, despite what airlines demand.

Nurcombe doesn’t recommend it, but she said passengers can probably stand on a Stelia Aerospace tray table without breaking it. They’re generally not designed to support a full human, but they are built to ensure a passenger can lean on them for support. Some passengers pull up on tray tables to stand.

Other parts of the seat are strong, too. Stella’s engineers know passengers also often stand by grabbing the headrest in front of them, so seat shells are reinforced. Moreover, television screens can be tougher than passengers might expect, allowing customers to grab them for support.

At Recaro, which makes business and premium class seats, engineers similarly plan for the worst-case scenario, said René Dankwerth, executive vice president of research and development. Recaro’s engineers know passengers sometimes stand on arm rests to pull a bag from overhead bins. They also know customers sometimes lose their balance while doing so, and bags fall from bins, whacking arm rests as they drop. Recaro’s products are designed to absorb the impact — and stay reliable.

“There will be no loose armrests [and], no flimsy table on our product,” Dankwerth said.

Honest Mistake? Or something else?

Often, passengers treat airplane interiors harshly in error. They’re unfamiliar with bins or tray tables, and perhaps they’re nervous or anxious, so they pull too hard. But other times, manufacturers wonder if airline passengers abuse seats on purpose.

For sure, they say, passengers do not treat them as gently as their own cars.

“Every time I get into an aircraft, I always wonder, ‘Who the hell sat in here before me? What have they done? They’ve trashed it,’ said Gareth Burks, managing director of rebel.aero, a seat-maker. “As a supplier to the industry, it’s really frustrating that you get hammered by quality control. They have to be perfect on day one. You just know by day two they look probably five years old — scratched and kicked.”

Sometimes, passengers remove items from seats, said Elijah Dobrusin, vice president of development and strategy at Lift by EnCore, an economy class seat supplier for the Boeing 737 and Boeing 787. Several years ago, he said, airlines realized some passengers were stealing seatbelts, perhaps to turn them into into belts for pants. (This unusual fashion accessory sell for $40 on the internet.)

Dobrusin said some seat-makers put a special lock on the seatbelts to ensure they’d stay on aircraft.

“People believe because they’ve paid for it they can do whatever they want to it,” he said. “They can step on it, they can try to break it, they can take off pieces of it and steal it. Which is absurd. People literally steal. Whatever thing is removable they’ll just take with them, which is quite weird.”

The good news is seats are safe, no matter what superficial damage passengers inflict on them. Even if they’re scratched or kicked repeatedly, they are designed to perform fine in a crash. (For certification, manufacturers musts prove their seats can withstand forces 16 times the force of gravity.)

They can last as long as the airplane, which can be longer than airlines want them. Many carriers switch out seats every decade or so, preferring newer ones that are almost always lighter than the previous version. Sometimes, they’re more comfortable than the previous generation, too.

“The product can fly for 20 years or even longer,” Dankwerth said, adding that seats can end up flying for a second or third airline.

That’s why reliability testing is so important. At Recaro, it means trying to “simulate the whole life of the product” before items go on a plane, Dankwerth said. A machine might place human-style pressure on an armrest a hundred thousand times, testing whether it can hold up to the rigors of three or four flights a day for two decades. Or it might open and close the tray table repeatedly — and fast.

Said Dankwerth: “Don’t get your fingers in between.”

Photo Credit: Seat manufacturer Recaro tries to "simulate the whole life of the product,” before it sells a new seat to airlines, an executive said. Pictured is the company's CL3710 economy class seat for long-haul flights. Recaro